Environmental conditions are known to cause many types of articles to rapidly degrade in quality and/or appearance. Degradation can occur due to interactions caused by many factors, including temperature, air, and light.
In the prior art, a clear plastic or glass display case is sometimes used to protect articles from the effects of air and temperature. With some display cases, the interior is sealed from the exterior air to limit effects on, and interactions with, the articles. With some display cases, the air within the case is removed and replaced by an inert gas, such as nitrogen, argon, or the like. Display cases of the type described above are found in many museums and exhibition halls, and are typically very large, very heavy, and very complicated to maintain.
Another environmental condition includes exposure to light, both natural and artificial, the effects of which can damage articles and/or any items in the vicinity of the articles, for example, items made of plastic and/or paper. Light comprises wavelengths of ultraviolet (UV), visible, and infrared energy. In the prior art, during periods of non-display and storage, articles and the other items in their vicinity are typically protected from the effects of light via use of display case filters and shields. To further shield against the effects of light, UV filters and/or opaque shields may be placed or adhered to exterior facing windows and interior lights that are within the display or movement area of the articles. A very informative web site that discusses the effects of visible light and other environmental factors on paper-based articles can be referred to for further information at www.stampsrart.com.
FIG. 7 is a chart showing the wavelengths and the amount of different types of interior and exterior light that potentially can be present within an interior of a windowed structure. Because interior artificial sources of visible light can be relied upon to provide sufficient illumination, the intense and destructive exterior sources of visible light can be totally blocked via shields or curtains placed over exterior facing windows. Although filters and/or shields can be used to minimize, if not eliminate, the UV and infrared components of both the artificial and natural light that is represented by FIG. 7, unfortunately, the goal of elimination cannot be applied in the same manner to visible light, which is needed by humans to visually see (i.e. to view articles with).
FIG. 8 is a table that illustrates relative effects of visible light. Although, as discussed above, the harmful effects of natural visible light from external sources can be eliminated, the harmful effects of indoor artificial visible light (which is usually left turned on for long periods of the time, even when there may be no viewers present) can nevertheless still cause significant damage and, thus, should always be considered when illuminating aesthetic, historical, rare, and/or valuable articles. Representative values of visible light measured by a light meter in lux under various types of indoor and outdoor settings include: about 100,000 lux present under direct outdoor natural sunlight, about 10,000 lux present under shaded outdoor natural sunlight, about 5000 lux present under indoor natural sunlight illuminated, about 1000 lux present under artificial halogen lights, a range of about 125 lux present under a 100 watt tungsten bulb measured at 3 feet, 1 lux present under a candle measured at one foot. The amount of visible light that can be present under indoor visible light is illustrated in FIG. 8 by a set of ranges that span 3000 lux to 30 lux. FIG. 8 illustrates that depending on a level of UV light also present, the possible damage that can be inflicted on an article exposed to visible light can span a 10,000 fold range. Exposure to visible light is cumulative such that cumulative exposures to 100-500 lux of artificial visible light (a range of amounts that are present in typical homes and offices) can cause sensitive material (for example art prints, stamps, etc) to begin fading in as few as one or two years. In other words, if preservation is a goal, the harmful effects of visible light, whether artificial or natural cannot be ignored. In the prior art, for example in museumlike settings, the amount of visible light from artificial sources is typically reduced by a simple technique of “lowering the lights,” which may include reduced wattage overhead lighting, or the flipping or turning of a switch to lower their intensity. Such techniques are highly dependent on someone, or something, being able to track and control the level of the light and, for this reason, in many instances, where artificial visible light has been identified as being of concern, the light is permanently kept in a dimmed condition. In the prior art, reduction of visible light is consequently achieved at a cost, for example, as occurs in the National Archives in Washington D.C., where treasures such as the Declaration of Independence are made much less enjoyable to view and study because of the uncomfortably low levels of illumination by artificial light (about 50 lux) that is used. Although museums such as The National Archives have the resources to be able to implement all that is needed to maintain their levels of light, because of cost and practicality private collectors typically and simply store their articles in containers and albums, and make them available for exhibition and viewing only intermittently. Unfortunately, when they do view or display their articles, private collectors do so by exposing them to the full effects of any indoor light and other environmental factors that may be present.
From time to time, it may be desired to more closely view a particular article, if not for personal pleasure, then for display, inspection, and the like. When prior art display cases are used for display or storage, such close inspection can normally be achieved only after a complicated and/or time-consuming process by which the display case is unsealed and/or opened. To minimize degradation of an article after removal from a prior art display case, additional filtering of light and climate control typically needs to be provided. With museums, the resources for implementation of additional climate control and light filtering are normally readily available. However, with private collectors, the apparatus and methods needed are typically too expensive and/or too difficult to implement. Thus, with private collectors, containers and albums to this day remain the storage method of choice. Because containers and albums in themselves provide no protection against light when open, and very little if any protection against other environmental effects, articles in private collections are typically subject to much more degradation than those in museums.
Two technologies that can be used to reduce transmission of light include photochromic and thermochromic technology. A property of photochromic material is that its transparency varies as a function of the amount of UV light it is exposed to. An example of photochromic technology known to those skilled in the art is that which is used to provide sunglass functionality, wherewith in the presence of UV light, eyeglass lenses can be made to darken, and in the absence of UV light, to lighten. Thermochromic technology is also known to provide light blocking functionality, but in response to changes in temperature and/or infrared wavelengths.
Although changes in transparency and opaqueness of photochromic material can be used to block visible light, a limitation arises in that the particular transparency of photochromic material can be made to change as a function of the UV light present. In situations where UV light has been prefiltered, for example by a UV filter placed over exterior facing windows and/or interior light fixtures, the photochromic functionality does not become activated. In such a case, where no, or very little, UV light is present, if it were desired to rely upon a photochromic material to shield an article from visible light, the photochromic material would fail to do so, and the article so shielded would remain exposed to its degrading effects.
As used herein, the term “light valve” describes a cell formed of two cell walls, usually constructed of plastic or glass, that are spaced apart by a small distance, with at least one wall being transparent. The walls have electrodes thereon, usually in the form of transparent, electrically conductive coatings. The electrically conductive coatings can be deposited on the walls in patterns so that different segments of the light valve can be selectively activated. Additionally, the electrodes on the walls may have thin, transparent dielectric overcoatings thereon. The cell contains a light-modulating element (sometimes herein referred to as an activatable material) which may, without limitation, be either a liquid suspension of particles, or all or a portion of the entire element may comprise a plastic film in which droplets of a liquid suspension of particles are distributed. As described hereinafter, moreover, a light valve may further comprise one or more additional layers to provide the light valve with additional capabilities.
The liquid suspension (sometimes referred to herein as a liquid light valve suspension, or simply as a light valve suspension) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement. Hence a beam of light passing into the cell is reflected, transmitted or absorbed depending upon the cell structure, the nature and concentration of the particles, and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state. Further in regard to the passage of light, the ΔT is defined as the difference in visible light transmission between the OFF and ON states. If desired, intermediate states can be achieved with the use of an appropriate voltage.
For many applications it is preferable for all or part of the activatable material, i.e., the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film in which droplets of liquid suspension are distributed are preferable to a liquid suspension because hydrostatic pressure effects, e.g., bulging associated with a high column of liquid suspension, can be avoided through the use of a film and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that, in a plastic film, the particles are generally present within very small droplets, and, hence, do not noticeably agglomerate when the film is activated with a voltage.
A light valve film (sometimes referred to herein as an SPD film), as that term is used herein, means a film or sheet, or more than one thereof comprising a suspension of particles intended to be used in a SPD light valve. Such a light valve film usually comprises a discontinuous droplet phase of a liquid or liquids comprising dispersed particles (i.e., a liquid light valve suspension), such discontinuous phase being dispersed throughout a solid continuous matrix phase, wherein the phases are enclosed within one or more rigid or flexible solid films or sheets. After curing, the combined aforesaid phases may be referred to as a cured SPD emulsion, which may constitute a part of a light valve film, sometimes also referred to as a film or a film layer. The light valve film and/or a laminate of the light valve film may also comprise one or more additional layers, such as, without limitation, a film, coating or sheet, or combination thereof, which may provide the light valve film with one or more characteristics or capabilities such as, (1) scratch resistance, (2) protection from ultraviolet radiation, (3) reflection of infrared energy, (4) electrical conductivity for transmitting an applied electric or magnetic field to the activatable material, (5) dielectric overcoatings, (6) color tinting, (7) acoustic control, and (8) an additional transparent electrically conductive layer which can heat all or part of the film or light valve if an electric current (AC or DC) is passed therethrough. As used herein the terms, “light valve film” or “SPD film” or “film” shall be understood to also include laminated films.
A common (but non-limiting) construction for an SPD film has five layers, namely, from one side to the other, (1) a first sheet of a polyethylene terephthalate (“PET”) plastic, conveniently 5-7 mils in thickness, (2) a very thin, transparent, electrically conductive coating of indium tin oxide (“ITO”) acting or capable of acting as an electrode, on the first sheet of PET, (3) a layer of cured, i.e., cross-linked SPD emulsion, usually 2-5 mils in thickness and (4) a second ITO coating acting or capable of acting as an electrode on (5) a second PET plastic substrate. As stated previously, additional layers that provide other functions may optionally be added to the multi-layer SPD film described above. Typically, copper foil, conductive fabric or the like are affixed to the electrodes so that they extend beyond the perimeter of the SPD film for convenient connection to a suitable voltage source. Furthermore, the SPD film can be laminated (see U.S. Pat. No. 7,361,252 assigned to the Assignee of the present invention/application), for example, between transparent hot melt adhesive films and/or glass or plastic sheets to provide strength and rigidity and to protect various parts of the combined unit from environmental stresses which may, otherwise, damage its performance characteristics.
Electric power to activate the light valve film can be obtained from any conventional or non-conventional source. For example, the assignee of the present invention/application has publicly demonstrated operation of an SPD film and light valve powered by photoelectric energy, which may be derived from solar energy or a suitable alternative light source, such as a lamp.
U.S. Pat. No. 5,409,734 exemplifies a type of non-cross-linked light valve film that is made by phase separation from a homogeneous solution. Light valve films made by cross-linking (curing) of emulsions are also known. The apparatus and methods of the present invention are specifically directed to the use of the latter type of film, i.e., film comprising a layer formed by cross-linking an emulsion, and to laminated films produced thereby. See, for example, U.S. Pat. Nos. 5,463,491 and 5,463,492 and U.S. Pat. No. 7,361,252, all of which are assigned to the assignee of the present invention/application. Various types of SPD emulsions, and methods for curing the same, are described in U.S. Pat. Nos. 6,301,040; 6,416,827 and 6,900,923, all of which are assigned to the assignee of the present invention/application. Such films and variations thereof may be cured through cross-linking brought about by exposing the films to (1) ultraviolet radiation, (2) electron beams or (3) heat. All of the patents and patent applications, as well as any other references cited herein are specifically incorporated by reference.
A variety of liquid light valve suspensions are well known in the art and such suspensions are readily formulated according to techniques well known to one having at least an ordinary level of skill in this field. The term “liquid light valve suspension”, as noted above, when used herein means a liquid suspending medium in which a plurality of small particles are dispersed. The liquid suspending medium comprises one or more non-aqueous electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer that acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
Liquid light valve suspensions useful in the present invention may include any of the so-called prior art liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media know in the art which are useful herein include, but are not limited to, the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175; 4,407,565; 4,772,103; 5,409,734; 5,461,506; 5,463,492 and 6,936,193, the disclosures of which are incorporated herein by reference. In general, one or both of the suspending medium or the polymeric stabilizer typically dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles, but which also dissolves in the non-aqueous liquid(s) which comprise the liquid suspending medium. Alternatively, there may be two more solid polymeric stabilizers serving as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer, such as nitrocellulose which, in effect, when dissolved, provides a plain surface coating on the particles, together with one or more additional types of solid polymeric stabilizer that, when dissolved, bond to or associate with the first type of solid polymeric stabilizer and which also dissolve in the liquid suspending medium to provide dispersion and stearic protection for the particles. Also, liquid polymeric stabilizers can be used to advantage, especially in SPD light valve films, as described for example in U.S. Pat. No. 5,463,492.
Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light absorbing or light reflecting in the visible portion of the electromagnetic spectrum.
Conventional SPD light valves have generally employed particles of colloidal size. As used herein the term “colloidal” generally means that the particles have a largest dimension averaging 1 micron or less. Preferably, most polyhalide or non-polyhalide particles used or intended for use in an SPD light valve suspension will have a largest dimension which averages 0.3 micron or less and more preferably averages less than one-half of the wavelength of blue light, i.e., less than 2000 Angstroms, to keep the light scatter extremely low.
As noted above, many items can be degraded due to exposure to light. As defined herein, “light” is defined as comprising any or all of infrared, visible and ultraviolet electromagnetic radiation except where otherwise specified. Light can cause degradation because it is composed of packets of energy, known as photons, which may cause changes in an object impacted thereby. As the wavelength of the light decreases, the energy of each individual photon increases. Hence, ultraviolet radiation is more potentially degrading than visible light which, in turn, is more potentially degrading than infrared (i.e., heat) radiation.
Objects that can be degraded by light are of many types. Without limitation thereto, such objects may comprise textiles, books, documents, paintings, other works of art, photographs, stamps, and other philatelic items, antiques and artifacts.
Many such objects are valuable. Some of them, in fact, may be considered as “priceless” by their owners or admirers. Accordingly, a methodology for displaying the objects while greatly reducing degradation from sunlight, lamps and other sources of light is urgently called for.
An attempt to describe the problem and to suggest some possible ways to ameliorate it is set forth in Published U.S. Patent Application No. 2007/0188873 A1 of Wardas published on Aug. 16, 2007 and subsequently abandoned. The disclosure set forth in the subject reference is incorporated herein by reference.
The amount of visible light that an object is exposed to may vary over a wide range, depending upon whether such exposure occurs outdoors or indoors and the type of illumination. While the method and apparatus of the present invention is not limited to use with visible illumination, such visible illumination may vary from more than 100,000 lux outdoors to as little as 1 lux indoors. Artificial light sources, on the other hand, are available in a number of different types including, without limitation, incandescent, fluorescent, halogen, tungsten and arc lamps, as well as light-emitting diodes.
In, for example, paragraphs [0009], [0010] and [0011] the Wardas publication proposes to protect objects from damage by light with any of three technologies which can function to block such light, namely photochromic, thermochromic and electrochromic technology. As noted above, photochromic technology is used, for example, in certain types of sunglasses in which the lenses darken when exposed to UV-containing light, such as sunlight. As also noted, thermochromic technology can block light when the materials respond to changes in temperature. Electrochromic technology uses an electric current to darken the electrochromic material, e.g., in an automobile rear-view mirror.
Unfortunately, however, each of the three aforesaid technologies have substantial deficiencies that render them impractical for protecting objects from degradation by light. As a result, to the best of the present inventors' knowledge, none of these technologies has yet been used in a commercially successful manner for such a purpose.
In order for a light-protectant technology to be of any real value, the present inventors believe that there are five criteria that a light-controlling device or system must meet, as follows:
1. In its darkest state, it must have a maximum of 20% visible light transmission, preferably a maximum of 10% and most preferably a maximum of 5%;
2. In its clearest state it must have a visible transmission exceeding 50%, and preferably exceeding 60%;
3. Blockage of ultraviolet radiation should be at least 95% and preferably greater than 99%;
4. A change from its clearest state to its darkest state, or vice-versa, must occur in 5 seconds or less; and
5. In its unpowered, i.e., OFF state it must be dark, so that in the event of a power outage the objects to be protected will still be protected from impingement by light.
Photochromic lenses require exposure to UV in order to darken, and because UV is the most degrading form of light affecting the sort of objects one typically wishes to protect, e.g., materials including but not limited to textiles, books, documents, paintings, other works of art, photographs, stamps, and other philatelic items, antiques and artifacts as noted above, permitting UV light to impinge upon such an object is inherently risky. Moreover, if no UV energy is present, the photochromic lenses or window material won't darken and, hence, the object will not be protected from the effect of visible light. In addition, photochromic lenses reportedly require 35 seconds to darken just from 70% light transmission to 30% light transmission, and a matter of minutes to darken or lighten completely. This is an unacceptably long time to wait.
On the other hand, a typical thermochromic material has a light transmission range of 14% to 69%, which would possibly be an acceptable range if there were no other problems with this technology. An important factor to keep in mind in this regard, however, is that the ambient temperature must be ranged above a comfortable indoor temperature in order to darken the material. Furthermore, in any event, about 20 minutes is required for the change from light to dark, or vice-versa, to occur. This is far too long for someone to wait for an exhibit to become visible when waiting, e.g., in front of a display case in a museum. Additionally, it allows too much time for visible light to impinge upon, and thus degrade, an object that it is desired to protect.
Electrochromic (EC) windows are reported to have a light transmission range of 2% to 62% and can reject infrared and UV radiation reasonably well. Thus, if they could turn on and off quickly, they might be acceptable for protecting objects from light. Unfortunately, this is not the case. The response speed of an electrochromic device is an inverse function of its size. Display cases, picture frames and partitions used in museums, for example, might range from perhaps one square foot to several square meters in size. An electrochromic window as small as one square foot would probably require at least 30 seconds to darken or lighten completely, and an electrochromic window measuring just one square meter in size would require tens of minutes to darken or lighten fully. Such an arrangement is, therefore, completely impractical. In addition, in the event of a power outage, an electrochromic window defaults to its most light transmissive state, which would expose the object that is supposed to be protected to the degrading effects of light.
As can be seen, then, from the discussion above, none of the three technologies discussed in the Wardas publication, i.e., photochromic, thermochromic and/or electrochromic technology, meet all of the above-described criteria (i.e., nos. 1-5) required for obtaining a useful degree of protection of objects that are to be protected from the damaging effects of light.